Daytime running lights augment vehicle safety by enhancing the visibility of a vehicle having daytime running lights. Thus, daytime running lights are becoming a more common feature on vehicles. Because of the recognized importance of daytime running lights on vehicles, Canada has legislated to require daytime running lights on vehicles that are sold in Canada.
In daytime running lights on a vehicle, a high-power output switching transistor delivers current to a high wattage daytime running light bulb filament on the vehicle. Referring to FIG. 1, a high-power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 102 is used to deliver current from a power source 104 to the filament of a daytime running light 106. The intensity (and thus the brightness) of the daytime running light is determined by the duty cycle of a switched DC voltage applied to the gate of the MOSFET 102.
Note, that a high power BJT (Bipolar Junction Transistor) may be used instead of the MOSFET 102 to deliver current from the power source 104 to the filament of the daytime running light 106. As would be apparent to one of ordinary skill in the art, the present invention may be used in conjunction with a MOSFET or a BJT or any other type of high power switching device from the description herein. The MOSFET 102 is shown as an example switching device. However, a MOSFET may be preferred as the switching device rather than a BJT because of the higher switching speed and lower resistance of a MOSFET when the MOSFET is turned on.
The power source 104 is typically from a battery system of the vehicle, and the battery system includes a fuse which blows and open-circuits the drain of the MOSFET 102 from the power source 104 when excessive current flows though such a fuse. However, a partial short load 108 may be coupled to the source of the MOSFET 102. The partial short load 108 has an impedance which is not low enough to blow the fuse of the battery system. Nevertheless, the impedance of the partial short load 108 may be low enough to cause high current to flow through the switching MOSFET 102.
Such high current flowing through the switching MOSFET 102 results in high heat and thus damage to the switching MOSFET 102. The damage to the switching MOSFET 102 results in an effective resistance 110. As high current continues to flow through the effective resistance 110, excessive overheating and/or even a fire may result in malfunction of the daytime running light system and could potentially be a further hazard to the vehicle.
Prior art protection circuits, for preventing damage to the switching transistor 102, control the current flowing though the switching transistor 102 by controlling the gate voltage if the switching transistor is a MOSFET or by controlling the base current if the switching transistor is a BJT (Bipolar Junction Transistor). U.S. Pat. No. 4,926,283 to Qualich, U.S. Pat. No. 4,595,966 to Huber et al, U.S. Pat. No. 4,750,079 to Fay et al., U.S. Pat. No. 5,272,392 to Wong et al., U.S. Pat. No. 5,390,069 to Marshall, U.S. Pat. No. 5,438,237 to Mullins et al., and U.S. Pat. No. 5,694,282 to Yockey, teach a protection circuit 112 which controls the current flowing through a MOSFET output transistor by correspondingly controlling the gate to source voltage of the MOSFET output transistor. U.S. Pat. No. 4,360,852 to Gilmore and U.S. Pat. No. 4,800,331 to Vesce et al., teach a protection circuit 112 which controls the current flowing through a BJT output transistor by correspondingly controlling the base current of the BJT output transistor.
Unfortunately, with the prior art protection circuit 112, a conductive path still exists from the power source 104 through the effective resistance 110 of the damaged MOSFET 102 to the partial short load 108. Even with the prior art protection circuit 112, excessive current may still flow through such a conductive path. Such excessive current may cause sufficient damage to the switching transistor 102 such that the daytime running light becomes inoperative. Additionally, such excessive current may cause excessive overheating and even a fire resulting in further hazard to the vehicle.
In addition, the prior art protection circuit 112 does not include a latching circuit to keep the daytime running light turned off once the switching transistor reaches an excessive temperature. Without the latching circuit, the daytime running light may flash on and off as the switching transistor alternately cools off and turns back on and then heats up and turns back off from the mechanism of the prior art protection circuit. Such flashing on and off of the daytime running light may be a traffic hazard on the road and also causes further degradation of the switching transistor 102.